The present invention provides improved surface smoothness in unsaturated polyester resin compositions that contain low-profile additives. More specifically, these unsaturated resin compositions contain low-profile additives and compatible compounds.
Unsaturated polyester resin compositions are finding increased use in the automotive industry as compositions from which component parts especially body panels can be molded. These compositions contain, in addition to the unsaturated polyesters, so-called low-profile additives which are thermoplastic polymers that act to prevent undesirable shrinkage as the composition is being molded into a thermoset article. Low-profile additives are added to unsaturated polyester compositions for the purpose of obtaining a composition which can be molded into thermoset articles, where the surfaces of the molded articles truly reflect the surface characteristics of the mold.
Two types of low-profile systems are commonly used commercially, one-pack and two-pack. In one-pack systems, the polyester, styrene, and low-profile additives are mutually compatible, i.e., no gross separation occurs when the mixture is allowed to stand. In contrast, two-pack systems form distinct phases if the resins are allowed to stand. These need to be mixed immediately prior to use. In either case phenomena occur that allow these resins to microscopically compensate for shrinkage.
It is the ability of low-profile resins to compensate for shrinkage that leads to the usefulness of these resins. Shrinkage compensation is largely a result of a micro-phase separation that occurs in these unsaturated polyester resin systems. The micro-phase separation occurs during cure for both one-pack and two-pack systems.
Prior to cure the low-profile additive is at least partly soluble in the polyester/styrene solution. As the polyester/styrene mixture crosslinks, the thermoplastic and copolymer become increasingly less compatible and a two-phase (domain-matrix) type morphology results. This micro-phase separation leads to the formation of porous structure as the opposing internal stresses of thermal expansion and polymerization shrinkage occur. In many unsaturated polyester resin compositions the porous structure is a result of microfracturing of the curing resins which gives rise to void formation. Unsaturated polyester resins have been developed which have essentially zero shrinkage and which, in fact, expand upon curing.
In addition to unsaturated polyester resins, sheet molding compound formulations contain other ingredients including chemical thickeners. In such systems, an alkaline material such as magnesium oxide or magnesium hydroxide is added to, for example, an uncured polyester along with fillers, glass fiber, and other standard materials. The alkaline material interacts with residual acidity in the polyester and usually the low-profile additive to build viscosity. This process is referred to as maturation and usually takes several days. If two-pack resins are used, care has to be taken to avoid gross phase separation. After maturation is complete, the thickened systems are handlable and can easily be placed into compression molds either by hand or by machine.
Although the use of low-profile additives as described as three component mixtures do effect some degree of improvement in anti-shrinkage characteristics of the unsaturated polyester compositions, it has been found that significant improvements could yet be made on surface smoothness and processing characteristics.